<script>
function closeLeftTabs() { 
	/*chrome.tabs.getSelected( null , function(tab) {  
		chrome.tabs.remove(tab.id, null);
		} ); 
		*/
	var curTab;
	chrome.tabs.getSelected( null , function(tab) {  
		curTab=tab;
	});
	chrome.tabs.getAllInWindow(null,function(tabs) {
		for (var i = 0; i < tabs.length; i++) {
			if (tabs[i].index<curTab.index){
				chrome.tabs.remove(tabs[i].id, null);
			}
		}
	});
} 

</script>

<div class="toolstrip-button" onclick="closeLeftTabs()">
  <span>Close Left Tabs</span>
</div>